1. Field of the Invention
The present invention relates to aspheric face form measuring to measure a form such as an aspheric face optical element or the like, for example.
2. Description of the Related Art
Generally, an interferometer has been widely used as a form measuring apparatus to perform form measuring of work where high precision is desired, such as optical parts of a semiconductor exposure apparatus or the like. An interferometer divides a beam of light emitted from a light source into a reference light and measurement light, recombines the measurement light and reference light that have been reflected from a subject face, and by detecting the combined light here (interference light) obtains a fringe pattern image. A phase is calculated from the optical strength of the fringe pattern image, and the phase there of is converted into height information, whereby form measurement of a subject face is performed.
Now, in the event of the above-described form measuring being performed, if the obtained fringe pattern image is a single color stripe overall (one color), the image thereof can be used to perform form measuring of the entire subject face. However, in the case that the subject face is an aspheric form, a sparse portion (portion where the stripe is one color) and a dense portion will arise in the fringe pattern. In the portion of the fringe pattern that is sparse, the curvatures of the spherical face and subject face of the measurement light are roughly the same, whereby the measurement light reflects off the subject face and returns by the same optical path, and other than a optical path difference that occurs by reflecting off the subject face, little optical path difference occurs as compared to the reference light. On the other hand, in the portion where the fringe pattern is dense, the optical path difference of the measurement light and referencing light, which results from the measurement light not being vertically incident as to the subject face, is not negligible, and if the measurement error is great, performing form measurement is difficult, which is complicated by a problem with sensor resolution.
Now, as described in Japanese Patent Laid-Open No. 2004-45168, various methods such as a so-called ring stitch method have been proposed as a method to measure an aspheric form. This ring stitch method changes the relative distance between a subject aspheric face and the origination point of measurement light (e.g. a pinhole or reference sphere face), and obtains a fringe pattern image while the fringe pattern moves a sparse portion (ring null region) in the diameter direction. From the fringe pattern from the obtained fringe pattern images, phase data of the sparse portions that can be used to measure form is obtained, the multiple points of phase data are connected, and transformed into height data, thereby obtaining the form of the subject aspheric face.
Now recently, EUV (Extreme Ultraviolet) light has been proposed as a light source to be used in semiconductor exposure apparatuses, and there is demand for measurement of the form of the subject with even higher precision.
In an EUV exposure apparatus, a projection optical system is configured with aspheric mirrors throughout, and a form measuring apparatus disclosed in PCT Japanese Translation Patent Publication No. 2008-532010 (P. 33, FIG. 24) has been proposed as a measuring device that can measure a high precision aspheric optical device such as an EUV exposure apparatus mirror at a precision greater than the precision in demand (e.g. 0.1 nm RMS).
The form measuring apparatus disclosed in PCT Japanese Translation Patent Publication No. 2008-532010 (P. 33, FIG. 24) has a basic configuration of a Fizeau interferometer which causes the reference light reflected off a reference sphere and the light that transmits the reference sphere and reflects off the subject aspheric face to interfere, and obtains the fringe pattern with two charge-coupled device (CCD) cameras. According to this form measuring apparatus, unlike the above-described ring stitch method, the difference between the fringe pattern phase of the aspheric face ring portion and the fringe pattern phase of the ring null region are used to express unevenness information in the vertical direction as to the subject aspheric face. Also, the horizontal coordinates, i.e. coordinate information on a plane vertical to the aspheric face axis (center axis) of the subject aspheric face are obtained from the scanning amounts of the subject aspheric face. Approximate values of the scanning amounts of the subject aspheric face can be measured with an end-measuring machine. Further, high precision measurements based on the fringe pattern phases are realized by using the fringe pattern phases on the aspheric axis portion of the subject aspheric face and correcting the measurement values of the end-measuring machine.
The measuring method of the form measuring apparatus disclosed in PCT Japanese Translation Patent Publication No. 2008-532010 (P. 33, FIG. 24) is called a fringe pattern zone scanning method. The features thereof include measuring the phase difference and scanning amount using the fringe pattern phase of an aspheric face axis portion and the fringe pattern phase of a ring null region, and the distance information from the end-measuring machine, and finding three-dimensional form information of the subject aspheric face by solving a predetermined equation. That is to say, the form of the portion of a subject aspheric face can be directly obtained from just a pair of fringe pattern phase and end-measuring information, whereby measurement information of adjacent steps is unnecessary. Therefore, for example, accumulation of measurement error which becomes a problem in form-measuring methods such as the above-described ring stitch method does not occur, and high precision measuring can be performed.
The method in Japanese Patent Laid-Open No. 2004-45168 has various innovations so that the error does not accumulate in the Z-axis direction, but error accumulation in the horizontal coordinates is not taken into consideration. Also, the inaccuracy of using the phase data for the entire ring null region is not taken into consideration.
On the other hand, the method in PCT Japanese Translation Patent Publication No. 2008-532010 (P. 33, FIG. 24) has high measurement precision, but of the fringe pattern phases in the ring null regions, only the forms for the vertical incident region where measurement light is incident vertical to the subject face can be measured. Therefore, in the case of measuring the form of the entire face of the subject face, just a circular cross-section form can be obtained from the phase information of one step, and in order to measure the form of the entire face at sufficient data density, a large number of steps is needed, leading to a problem in that the measuring takt time becomes that much longer.